Simpletransreceiver ~ Part 22
Thoughts on the Control Circuitry
We have all heard that a long journey must start with the first step ( I guess 22 parts qualifies this as a long journey). With the Holidays my bench time has been somewhat limited but I did want to provide some of my thoughts on a control circuit for the transreceiver in the event there are those who want to forge ahead.
Perhaps one of the most critical items to make the receiver and transmitter play as a unit is how to control the transition from receive to transmit AND to accomplish that end without hiccups, burps, spurious artifacts, hot switching, "kerchunks" and on an on. Those who have followed my projects will find that I like to recycle circuits from prior projects chiefly so I am not continuously trying to reinvent the wheel and secondly since they are proven their performance is known.
The solid state control circuit that you will see shortly was a result of a project I did in 2009 where everything was switched with relays. At that time the frequency control in the dual conversion transceiver (my solid state version of the Heathkit HW-100) was a PTO from a Ten Tec Triton IV. There was such a terrific back emf from all of the relays that the frequency would change by a few kHz every time there was a TR.
Below is the breadboard for that project. If you look in the upper portion of the photo near the center you can see a huge 4PDT Power Relay and somewhat obscured is the RF boards that had relay selected Band Pass Filters and another board that relay selected Low Pass Filters. There was also a TR relay to redirect the antenna to the right portion of the circuit. So after a week or so of trying to resolve this issue I came up with a solid state switching scheme which has been refined with several iterations and will most likely need work for the Simpletransreceiver BUT it is a place to start.
Next is the schematic of the baseline control circuit. STOP --I already see many of you sending me an email that since I have an Arduino why not use that to do all of the control. I am aware of that but that would require more development time. So why not use a proven hardware solution!
(Some day I will share my Arduino Linear Amp control scheme that senses SWR, Power Source Voltage, Over Voltage, Over Temperature and sequenced step start of the power supply --so I do know how to do it --but an overkill for this application.)
For those who may have seen my JABOM transceiver (Just A Bunch Of Modules), this was the circuit used for that project. Essentially the PTT triggers the optoisolator (4N35) to toggle the 7400 IC wired as an Inverter. So OK use your favorite inverter chip --this is what I had in the Junk Box! With the wiring as shown the + 12 VDC R is always "On". BUT when you key the PTT this toggles the SN74LS00 to the other state and there is a transition from +12 VDC R to +12 VDC T. So long as the PTT is held in --you have + 12 VDC T.
The TIP32C PNP devices are good for a couple of amps --if you want to switch bigger loads then circuits with power MOSFET's would be the order of the day. So please no emails that "your circuit doesn't work or why did you do this or that?" This is a starting place and the circuit WORKS! You are welcome to make any changes.
There have been embellishments of this same circuit where I used a NE555 timer that was keyed and with the inclusion of one relay added a timed closure of the PTT such as you would have while sending CW or for a Tune function. It was successfully used in my KWM-4 SSB/CW transceiver. There is no difference between using 1N914's or 1N4148's -- I am merely trying to head off a flood of inquiries! Also read the schematic and notes carefully -- I have received emails about what is the * after the SN74LS00 and what do you do with the unused pins -- the schematic is clear --GROUND THEM!
[Caution: This control circuit is a starting point to demonstrate a way to electronically switch most of the circuits from receive to transmit. It is envisioned that it will work pretty much as shown BUT not having been tested with the receiver and transmit boards and other ancillary equipment such tests may result in some modifications. Thus the material is being presented now to show that there is plan being set forth to control the two major elements. Since this is a transreceiver and not a transceiver per se there may be other additions required. Bottom line "Heads Up".]
I have not detailed the interconnection to the receiver or transmitter circuits --remember this is a starting place. But basically the plan is to identify modules that would always remain powered regardless of Transmit or Receive such as the Arduino/AD9850 and probably the Audio amp stage. What will be switched are circuits such as the Rx RF AMP and Rx IF AMP & Product Detector and a maybe on the BFO. On the transmit side the Microphone Amp and the transmitter specific circuits and final the antenna change over. In work is a block diagram of the control circuits but some of that will be refined after hookup.
This information is being provided now so that those who want to start the control circuits have most of the information needed. Below is the Low Pass Filter Information. The inductors have been wound using a standard number of turns and the cutoff made slightly beyond 8 MHz and the second harmonic suppression is almost unbelievable. In/Out is 50 Ohms.
Many are endeared to the BITX IRF510 Final amplifier --I am not and a future post will include a "real RF Device" final amplifier as used in the JABOM, ZIA and KWM-4 transceivers.